1. Field of the Invention
This invention relates to a communication technique for extending the band by distributing data to a plurality of paths. More particularly, the present invention relates to a communication apparatus, a communication system, an absent packet detecting method and an absent packet detecting program for detecting any packet loss (absence) very quickly by means of only a series of sequence number in a multi-path environment where a transmitter and a receiver are connected to each other by way of a plurality of networks when no inversion of sequence arises in any of the networks.
2. Description of the Related Art
Communication techniques of branching a single communication flow of date being used between a transmitting terminal and a receiving terminal into a plurality of flows and restoring the original flow are known. They include a proposed method of arranging gateways respectively at the first LAN (local area network) to which the transmitting terminal belongs and the second LAN to which the receiving terminal belongs, allocating the data of the TCP (transmission control protocol) transmitted from the transmitting terminal to communication paths at the gateway of the first LAN on the basis of units of packets and restoring the data at the gateway of the second LAN, correcting the inverted sequence of the packets received from the plurality of communication paths according to the sequence numbers of the TCP (see, for example, Patent Document 1: JP 2000-261478A).
They also include a proposed method of utilizing efficiently a plurality of communication lines and improving the capacity factor of the communication lines by adding functions to the TCP of terminals so as to utilize a plurality of TCP connections for communications that hitherto employ only a single TCP connection (see, for example, Patent Document 2: JP 2003-110604A). With such a method, the communication that is realized by a single communication flow from the transmitting terminal to the receiving terminal is divided into a plurality of communication flows and a data is transmitted in parallel. When data is transmitted from a transmitting terminal to a receiving terminal, the communication protocol of the transmitting terminal divides the data of the single communication flow into a plurality of communication flows and adds a new header to each TCP/IP packet as restoration information for restoring the plurality of communication flows produced by the division to the original single communication flow (by using two series of sequence numbers) in order to transmit the data by way of a plurality of communication flows. The communication protocol of the receiving terminal restores the original single communication flow from the plurality of communication flows by referring to the restoration information of the data it receives.
However, the above-described known methods are accompanied by problems.
The first problem is that a packet loss is detected only slowly.
The communication method described in the Patent Document 1 is designed to determine the absence of a packet by seeing the abnormal sequence, if any, on the basis of an assumption that the TCP of a terminal is employed on a single communication path. Therefore, an erroneous detection of an absent packet (a packet loss or an abandoned packet) occurs when a plurality of communication paths are employed for communication and inversion of sequence arises due to the difference of delay time of the communication paths. In other words, the method cannot discriminate an abandoned packet and inversion of sequence. When any absent packet due to an abnormal sequence is neglected in order to avoid this problem, detection of a packet loss becomes a slow operation because an absent packet can be determined only by means of retransmission timeout.
The TCP employs detection of a packet loss as trigger for retransmission. Therefore, when a packet loss is detected only slowly, the retransmission also starts only slowly to reduce the throughput. Then, the potential of the communication paths cannot be exploited satisfactorily.
The second problem is a rise of communication cost.
The communication method described in the Patent Document 2 requires headers arranged in two stages and two series of sequence numbers. Two series of sequence numbers by turn requires a complex arrangement for packet transfers and system management to raise the communication cost.